BE 9.1 does not support the VXA-172 or VXA-320 drives natively. You need to use the VXA Tool from Exabyte's website to change the inquiry string on the drive so it reports as a VXA-2 or just upgrade to 10.x.
If you go the route of emulation, technically, Veritas / Symantec won't support you.
Either way, you should get that tool from the Exabyte and update the drive firmware.
VXA Tool for Windows:
ftp://ftp.exabyte.com/pub/tools/vxa/VXATool_WIN_V2.45.zip
VXA-320 firmware:
ftp://ftp.exabyte.com/pub/firmware/vxa/VXA320_V13206.zip